Workpiece inspection method

ABSTRACT

A method of inspecting an artefact using a non contact measurement mounted on a coordinate measuring apparatus. An artefact is measured first with a contact probe ( 28 ) and then with a non contact probe ( 32 ). An error map or function is generated ( 34 ) which corresponds to the difference between the measurements taken with the contact and non-contact probes. This error map or function may be used to calibrate the probe. Alternatively subsequent artefact may be measured with the non contact probe ( 36 ) and the error map or function used to correct the measurements ( 38 ).

This invention relates to a method of inspecting the dimensions ofworkpieces using coordinate measuring apparatus. Coordinate measuringapparatus includes, for example, coordinate measuring machines (CMM),machine tools, manual coordinate measuring arms and inspection robots.In particular, the invention relates to a method of inspecting thedimensions of a workpiece using a non-contact probe.

It is common practice after workpieces have been produced, to inspectthem on a coordinate measuring machine (CMM) having a quill onto which aprobe is mounted which can be driven in three orthogonal directionsX,Y,Z within a working volume of the machine.

Workpiece measuring probes may be divided into contact probes andnon-contact probes. Contact probes comprise a housing with aworkpiece-contacting stylus deflectable with respect to the housing.There are two main types of contact probe. In a touch trigger probe, thestylus is deflected from a rest position to cause a signal whichindicates that the stylus has touched the surface of the workpiece.Alternatively a contact probe may comprise a scanning probe in which thedeflection of the stylus is continuously measured as the stylus is movedalong the surface of the workpiece.

Non-contact probes are positioned close to the surface of the workpiecewithout touching. The probe detects the proximity of the surface using,for example, capacitance, inductance or optical means.

Both contact and non-contact probes suffer from the disadvantage thatscanning at a high speed causes dynamic errors in the system due toinertia.

Inaccuracies caused by the dynamic deflection of the probe may bereduced by causing the probe to travel very slowly.

Our previous U.S. Pat. No. 4,991,304 discloses a method of inspecting aseries of workpieces using a coordinate measuring machine (CMM) in whicha contact probe is first calibrated or datumed for each intendeddirection of probing movement by touching it at a slow speed against areference object such as a datum ball to provide a set of correctionoffsets which are stored in the computer and used for subsequentmeasurement.

The first workpiece to be measured is put on the CMM table and a set ofpoints on the surface of the workpiece are measured at a slow speed toallow accurate readings to be taken. Measurement of the first workpieceis then repeated at a fast speed. The difference between the slow speedreadings and the fast speed readings is calculated and stored. Thestored error value for each measured point takes into account thedynamic deflections of the machine structure at the fast speed.

The next workpiece to be measured is set up on the CMM table andreadings are taken at the fast speed. At this speed the readings areinaccurate but repeatable. Each fast reading is adjusted by adding thecorresponding stored error value and thus compensating for errorsinduced by fast reading.

This method has the advantage that a whole series of nominally identicalworkpieces can be measured at fast speed by making a dynamic error mapfrom only one workpiece.

However, a disadvantage of this method is that fast contact scanning ofa workpiece causes significant wear of the stylus tip of the probe.

Non-contact probes have the advantage that as there is no contactbetween the probe and workpiece, there is no wear of the probe.

Another advantage of non-contact probes is that there are no errors dueto measurement force. In contact probes this is the force exerted by theprobe on the workpiece and causes measurement errors due to bending ofthe stylus, coordinate positioning apparatus and deformable parts of theworkpiece.

A further advantage is that non-contact probes have a higher surfacesensing bandwidth that contact probes and thus provide more responsivemeasurement when scanning or measuring a workpiece at higher speed.

However use of a non-contact probe also has several disadvantages. Theprobe may have radial errors due to the manufacturing process whichresults in variations of the measurement data for measurements taken atdifferent angles around the probe. This could be corrected for by anelaborate calibration.

In addition non-contact probes, such as inductance and capacitanceprobes, are influenced by the geometry of the part being measured andmeasurement data may vary, for example, between a straight and curvedsurface at the same distance from the probe. The surface finish of thepart may also affect the measurement data from a non-contact probe,particularly for optical probes.

The present invention provides a method of inspecting an artefact usinga coordinate measuring apparatus in which an artefact-sensing probe ismoved into a position-sensing relationship with each artefact and aposition reading taken, the method comprising the following steps in anysuitable order:

-   -   measuring said artefact with an artefact-sensing probe in        contact mode;    -   scanning said artefact with an artefact-sensing probe in        non-contact mode;    -   generating an error map or function corresponding to the        difference between the measurements taken with the artefact        measuring probe in contact mode and the artefact measuring probe        in non-contact mode; and    -   using the error map or function to correct subsequent        measurements with the artefact-sensing probe in non-contact        mode.

The step of measuring said artefact with an artefact-sensing probe incontact mode may comprise scanning said artefact.

The artefact may be scanned with the artefact-sensing probe innon-contact mode with the artefact-sensing probe at a nominal offsetfrom the artefact.

The error map or function may be used to calibrate the artefact-sensingprobe in non-contact mode to thereby produce a radial correction for anominal distance and direction of the artefact-sensing probe relative tothe artefact surface.

The method may also comprise the steps of: measuring subsequentartefacts with the artefact-sensing probe in non-artefact mode andcorrecting the artefacts using the error map or function.

The subsequent artefact may comprise a workpiece. The initial artefactmay comprise a workpiece substantially the same as the subsequentartefact. The initial artefact may be different from the subsequentartefact.

The same artefact-measuring probe may have both contact and non-contactmodes, or these may be provided by two different probes.

The error map enables the non-contact scan to be corrected formeasurement errors, and thus the probe does not need an elaboratecalibration.

If the first artefact is substantially identical to the subsequentartefacts, then the error map or function also corrects for measurementerrors of the non-contact probe caused by the geometric influence of theartefact.

In a subsequent embodiment of the invention, the artefact is measuredthe first time at a slow speed and the artefact is measured the secondtime at the speed of measurement of subsequent artefacts. Preferably thespeed of measurement of subsequent artefacts is a fast speed.

This method reduces wear on the contact stylus tip and compensates forboth dynamic speed errors and measurement errors of the non-contactprobe at the same time.

The artefact may be measured the first time with a contact probe on ahigh accuracy reference machine, for example a CMM in a calibrationlaboratory. The artefact may then be measured the second time with thenon-contact probe on a repeatable in-line (e.g. shop floor) coordinatemeasuring apparatus. A machine tool, when used for measuring an artefactwith a probe would comprise a coordinate measuring apparatus. The errormap or function generated may be used to correct the measurements ofsubsequent artefacts measured using the non-contact probe and in-linecoordinate positioning machine. The error map or function may thereforeaccommodate one or more of the following errors: non-contact probemeasurement errors, measurement errors due to the surface geometry ofthe artefact, geometric errors of the in-line machine and dynamic errorsof the in-line system.

The artefact-measuring probe in non-contact mode may comprise forexample an optical probe, a capacitance probe or an inductance probe.Generally these sensors are one-dimensional or scalar sensors and thusit is an advantage to use them in predefined path measurement mode.However this is not a requirement as the probes can also be used inunknown path mode.

Preferably the measurements of the workpiece from the contact probe areused to calculate a path for the non-contact probe to follow, especiallyif the surface of the artefact is non-prismatic/geometric.

A second aspect of the present invention provides apparatus forinspecting an artefact using a coordinate measuring apparatus and atleast one artefact sensing probe, the apparatus comprising a controlleradapted to perform the following steps in any suitable order:

-   -   (a) measuring said artefact a first time with an        artefact-sensing probe in contact mode;    -   (b) measuring said artefact a second time with an        artefact-sensing probe in non-contact mode;    -   (c) generating an error map or function corresponding to the        difference between the measurements taken with the artefact        measuring probe in contact mode and the artefact measuring probe        in non-contact mode;    -   (d) measuring subsequent artefacts with the artefact measuring        probe in non-contact mode; and    -   (e) correcting the measurements of subsequent artefacts using        the error map.

Preferred embodiments of the invention will now be described by way ofexample, with reference to the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a contact probe mounted on a coordinatemeasuring machine;

FIG. 2 is a schematic diagram of a non-contact probe mounted on acoordinate measuring machine;

FIG. 3 is a schematic diagram showing a contact probe scanning a bore ofa workpiece;

FIG. 4 illustrates the paths of the contact probe and non-contact probewhen scanning the bore of FIG. 3;

FIG. 5 is a flow chart illustrating the scanning method;

FIG. 6 is a flow chart illustrating a scanning method according to thesecond embodiment of the invention; and

FIG. 7 illustrates a non-contact probe mounted on an articulating head.

The coordinate measuring machine shown in FIG. 1 comprises a machinetable 12 on which a workpiece 16 may be placed. Preferably this is doneby automatic means (not shown) which places each of a succession ofsubstantially nominally identical workpieces from a production run in atleast nominally the same position and orientation on the table. Ananalogue probe 14 having a deflectable stylus 18 andworkpiece-contacting tip 20 is mounted on a quill 10 of the machinealthough other types of contact probes (including touch trigger probes)may also be used. The quill 10 and probe 14 may move in X,Y and Zdirections under the action of X,Y and Z drives controlled by acomputer. X,Y and Z scales (which include counters for the outputs ofthe scales) show the instantaneous coordinates of the position of thequill on which the probe is mounted in three dimensions. Signals fromthe probe indicating the deflection of the probe stylus are combinedwith the readings from the X,Y and Z scales of the CMM to calculate theposition of the stylus tip and thus the surface of the workpiece.Alternatively, with a touch trigger probe a signal indicating that theprobe has contacted the surface of the workpiece freezes the scales andthe computer takes a reading of the coordinate of the workpiece surface.

As thus far described, the machine is conventional. The computercontains a programme which causes the probe to scan the surface of theworkpiece or for a touch trigger probe to contact the surface of theworkpiece at a plurality of different points sufficient to take all therequired dimensions of the workpiece for the inspection operationrequired.

The analogue and touch trigger probes described both comprise contactprobes in which the stylus 18 of the probe 14 is deflected on contactwith the workpiece.

FIG. 2 shows a non-contact probe 22 mounted on the quill 10 of acoordinate measuring machine, the non-contact probe 22 may comprise, forexample, an optical probe, capacitance probe or inductance probe. As thequill 10 moves the probe 22 in a path around the workpiece 16, the probedetects the distance between itself and the surface of the workpiece.Signals from the probe are combined with the readings from the X,Y and Zscales of the CMM to calculate the position of the surface of theworkpiece.

Referring to FIG. 5, the following procedure is used in the presentinspection method. An artefact, such as a calibration artefact or aworkpiece, is set up on a coordinate positioning machine 26, for examplea CMM, and scanned or measured with a contact probe 28, for example ananalogue probe. This contact probe is calibrated for static errors byconventional means, for example as described in U.S. Pat. No. 4,991,304in which a set of correction offsets is calculated by touching the probeat a slow speed against a reference object, such as a datum ball, in aplurality of directions. These correction offsets are then used tocorrect all subsequent measurements.

The contact probe is exchanged for a non-contact probe, for example aninductance probe. The workpiece is then scanned or measured using thenon-contact probe 32.

An error map or function is generated 34 from the difference between theresults from the contact scan and the non-contact scan.

Subsequent artefacts are now placed on the CMM and scanned or measuredusing the non-contact probe 36.

Measurement data corresponding to the subsequent artefacts taken withthe non-contact error map may thus be corrected using this error map orfunction 38. This method enables the use of an uncalibrated non-contactprobe to be corrected for measurement errors.

Certain features of a workpiece, such as different surfaces and corners,may have an effect on the measurements from a non-contact scan,particularly with inductance and capacitance probes. An advantage of thepresent method is that errors due to these effects which may occurduring the non-contact scan are corrected by the error map or functionas the measurement data from the contact scan is not effected by thesegeometric influences. Thus measurements taken using the non-contactprobe of subsequent workpieces having the same geometric features willalso be corrected for these geometric influences.

The workpiece may be scanned using a probe which operates in bothcontact and non-contact modes. A single probe may be a combined touchtrigger, contact scanning and non-contact probe. For example a combinedtouch trigger and non-contact probe may follow a path around theworkpiece taking touch trigger points and then move around the path asecond time taking non-contact measurements. Alternatively a combinedtouch trigger and non-contact probe may be brought into contact with asurface of the workpiece to obtain a trigger point and then reversedaway from the surface to enable a non-contact measurement to be taken.This method allows the non-contact probe to be calibrated.

A second embodiment of the invention will now be described withreference to FIG. 6. In this embodiment, a workpiece having an unknownsurface, from a series of workpieces to be measured is set up on the CMM40 and scanned or measured at a slow speed with the contact probe 42. Atthis slow speed, the dynamic errors of the system are negligible. Forexample, typically the speed may be <20 mm/s.

The workpiece is then scanned using the non-contact probe 44. This scanis carried out at a speed at which the subsequent workpieces will alsobe scanned. This is a fast speed to facilitate high speed inspection.The fast speed is preferably greater than 20 mm/s, for example it maytypically be 100 mm/s.

As before, an error map or function is generated 46 corresponding to thedifference between the results from the slow speed contact scan and thefast speed non-contact scan.

Subsequent workpieces in the series of workpieces are set up on the CMMand scanned by the CMM using the non-contact probe 48. The data relatingto the subsequent workpieces is corrected by the error map 50. Thesubsequent parts are measured at substantially the same speed as before,i.e. the fast speed of the non-contact probe.

FIG. 3 illustrates a bore 24 of a workpiece 16 being scanned with acontact probe 14. The path of the workpiece-contacting probe 14 whenscanning the bore 24 is shown as A in FIG. 4. This profile accuratelydepicts the surface of bore 24 as the contact probe is calibrated toeliminate static errors and the bore is scanned slowly to reduce dynamicerrors.

The data collected from the contact scan may be used to calculate a pathC along which the non-contact probe travels to scan the bore 24. Thispath C is offset from profile A.

The surface of the bore 24 as measured by the non-contact scan is shownby profile B. This profile B may less accurately depict the surface ofthe bore 24 than profile A as the non-contact probe has not beencalibrated for either static or dynamic errors or radial errors due tomanufacture and geometric features of the surface. The differences dbetween profiles A and B are used to calculate error values by whichsubsequent non-contact scans are corrected.

This method thus has the advantage that both dynamic and static errorsof the non-contact measurement method are compensated for. Dynamic speederrors are compensated for by the initial slow scan with the contactprobe and static errors are compensated for by the initial slow scanbeing carried out with a calibrated probe.

Non-contact probes are usually one-dimensional and it is thus preferableto calculate the path of the non-contact scan to follow. Themeasurements taken by the contact probe may be used to calculate thepath for the non-contact probe to follow. For example this path may beoffset from the measured surface of the artefact a certain distance X.

It may not be necessary to use data collected from the contact scan tocalculate the path of the non-contact scan. For example, if theworkpiece has nominally predefined features, the non-contact scan can beeasily ascertained from these features. Furthermore, if amulti-dimensional non-contact sensor is used, the workpiece may beeasily measured using unknown path techniques.

The invention is not limited to the coordinate measuring apparatusproviding movement of the probe relative to the artefact along threeorthogonal axes. For example, the coordinate measuring apparatus maycomprise a rotary table on which the artefact is placed which allows theartefact to be rotated relative to a probe.

Alternatively, or additionally, the probe may be mounted on anarticulating head which may have one or more rotational degrees offreedom. FIG. 7 illustrates a non-contact probe 22, for example aninductance probe, mounted on an articulating head 52 which is in turnmounted on a spindle 10 of a coordinate measuring machine. Thearticulating head 52 comprises a fixed housing 54 which is mounted tothe machine spindle 10. A second housing 56 is rotatable with respect tothe first housing 54 about an axis A1. The non-contact probe 22 ismounted rotatably to the second housing 56 and is rotatable about asecond axis A2, orthogonal to the A1 axis. The artefact may be measuredby the probe mounted on such an articulating head by rotation of theprobe by the head or a combination of rotation and translation of thehead by the coordinate measuring apparatus.

1.-22. (canceled)
 23. A method of inspecting an artefact using a coordinate measuring apparatus in which an artefact-sensing probe is moved into a position-sensing relationship with each artefact and a position reading taken, the method comprising the following steps in any suitable order: measuring a surface of an artefact with an artefact-sensing probe in contact mode; measuring said surface of the artefact with an artefact-sensing probe in non-contact mode; generating an error map or function corresponding to the difference between the measurement taken with the artefact-sensing probe in contact mode and the artefact-sensing probe in non-contact mode; and using the error map or function to correct subsequent measurements with the artefact-sensing probe in non-contact mode.
 24. A method according to claim 23 wherein the step of measuring said surface of the artefact with an artefact-sensing probe in contact mode comprises scanning said artefact.
 25. A method according to claim 23 wherein said surface of the artefact is measured with the artefact-sensing probe in non-contact mode with the artefact-sensing probe at a nominal offset from said surface of the artefact.
 26. A method according to claim 23 wherein the error map or function is used to calibrate the artefact sensing probe in non-contact mode to thereby produce a radial correction for a nominal distance and direction of the artefact sensing probe relative to said surface of the artefact surface.
 27. A method according to claim 23, comprising the additional steps of: measuring subsequent artefacts with the artefact measuring probe in non-contact mode; and correcting the measurements of subsequent artefacts using the error map or function.
 28. A method according to claim 23 wherein the artefact-sensing probe in contact mode and the artefact-sensing probe in non-contact mode comprise a single artefact-measuring probe with both contact and non-contact modes.
 29. A method according to claim 23 wherein the artefact-sensing probe in contact mode and the artefact-sensing probe in non-contact mode comprise separate probes.
 30. A method according to claim 23 wherein said surface of the artefact is measured with the artefact-sensing mode in contact mode at a slow speed and with the artefact-sensing mode in non-contact mode at the desired speed of measurement of subsequent artefacts.
 31. A method according to claim 30 wherein the speed of measurement of subsequent artefacts is a fast speed.
 32. A method according to claim 23 wherein said surface of the artefact is measured with the artefact-sensing probe in contact mode using a high accuracy reference co-ordinate measuring apparatus and said surface of the artefact is measured with the artefact-sensing probe in non-contact mode using a repeatable co-ordinate measuring apparatus.
 33. A method according to claim 23 wherein the measurements of said surface of the artefact gained from measurement with the artefact-sensing probe in contact mode are used to calculate a path for the artefact-sensing probe in non-contact mode to follow.
 34. A method according to claim 23 wherein the path for the artefact sensing probe in non-contact mode is ascertained using predefined features of the artefact.
 35. A method according to claim 23 wherein the step of measuring a surface of said artefact with the artefact sensing probe in non-contact mode comprises scanning said surface of the artefact.
 36. Apparatus for inspecting an artefact using a coordinate measuring apparatus and at least one artefact sensing probe, the apparatus comprising a controller adapted to perform the following steps in any suitable order; (a) measuring a surface of an artefact with an artefact-sensing probe in contact mode; (b) measuring said surface of an artefact with an artefact-sensing probe in non-contact mode; (c) generating an error map or function corresponding to the difference between the measurements taken with the artefact measuring probe in contact mode and the artefact measuring probe in non-contact mode; (d) measuring subsequent artefacts with the artefact measuring probe in non-contact mode; and (e) correcting the measurement of subsequent artefact using the error map or function.
 37. Apparatus for inspecting an artefact using a coordinate measuring apparatus and at least one artefact sensing probe, the apparatus comprising a controller adapted to perform the following steps in any suitable order: measuring the surface of an artefact with an artefact-sensing probe in contact mode; measuring said surface of the artefact with an artefact-sensing probe in non-contact mode; generating an error map or function corresponding to the difference between the measurement taken with the artefact-sensing probe in contact mode and the artefact-sensing probe in non-contact mode; and using the error map or function to correct subsequent measurements with the artefact-sensing probe in non-contact mode.
 38. Apparatus according to claim 37 wherein the step of measuring said artefact with an artefact-sensing probe in contact mode comprises scanning said artefact.
 39. Apparatus according to claim 37 wherein said surface of the artefact is measured with the artefact-sensing probe in non-contact mode with the artefact-sensing probe at a nominal offset from said surface of the artefact.
 40. Apparatus according to claim 37 wherein the error map or function is used to calibrate the artefact sensing probe in non-contact mode to thereby produce a radial correction for a nominal distance and direction of the artefact sensing probe relative to the artefact surface.
 41. Apparatus according to claim 37, comprising the additional steps of: measuring subsequent artefacts with the artefact measuring probe in non-contact mode; and correcting the measurements of subsequent artefacts using the error map or function.
 42. Apparatus according to claim 37 wherein said surface of the artefact is measured with the artefact-sensing mode in contact mode at a slow speed and with the artefact-sensing mode in non-contact mode at the speed of measurement of subsequent artefacts.
 43. Apparatus according to claim 42 wherein the speed of measurement of subsequent artefacts is a fast speed.
 44. Apparatus according to claim 37 wherein the measurements of the surface of the artefact gained from measurement with the artefact-sensing probe in contact mode are used to calculate a path for the artefact-sensing probe in non-contact mode to follow.
 45. Apparatus according to claim 37 wherein the path for the artefact sensing probe in non-contact mode is ascertained using predefined features of the artefact.
 46. Apparatus according to claim 37 wherein the step of measuring said artefact sensing probe in non-contact mode comprises scanning said surface of the artefact. 